1. Field of the Invention
The present invention relates to a wireless communication system such as a wireless LAN (Local Area Network) for communicating between a plurality of wireless stations, and in particular, to a wireless communication system for realizing broadband wireless transmission in communication environments such as in homes.
Further more specifically, the present invention relates to a wireless communication system for expanding transmission capacity by communication in which spatial multiplexing is utilized (MIMO communication), pairing a transmitter having a plurality of antennas with a receiver having a plurality of antennas. In particular, the present invention relates to a wireless communication system for transmitting a large amount of data in series at a time with a MIMO communication scheme, avoiding deterioration of decoding characteristics.
2. Description of the Related Art
Canonical standards concerning wireless networks can include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see non-patent document 1), Hiper-LAN/2 (e.g., see non-patent document 2 or 3), IEEE802.15.3, and Bluetooth communication, for example. IEEE802.11 has enhanced standards such as IEEE802.11a (e.g., see non-patent document 4), b, and g depending on differences of wireless communication schemes and frequency bands.
IEEE802.11a supports a modulation scheme for achieving a maximum communication speed of 54 Mbps. However, there is desired a standard for realizing a higher bit rate for the communication speed. For example, in IEEE802.11n, with the aim of establishing a wireless LAN technology for realizing a high speed exceeding an actual throughput of 100 Mbps, the next generation of wireless LAN standards is being developed.
As a technology for realizing a higher speed of wireless communication, MIMO (Multi-Input Multi-Output) communication is coming to attention. This is a technology for expanding transmission capacity and achieving improvement in communication speed by realizing a spatial multiplexing transmission channel (hereinafter also referred to as “MIMO channel”) with a plurality of respective antenna elements at a transmitter and a receiver. In the MIMO communication, good frequency utilization efficiency is obtained due to utilization of spatial multiplexing.
In a MIMO communication scheme, the transmitter distributes transmission data to a plurality of antennas and transmits it through a plurality of virtual MIMO channels, and the receiver obtains reception data by processing signals received by a plurality of antennas. The MIMO communication scheme utilizes channel characteristics as described and differs from a mere transmission/reception adaptive array. That is, the transmitter performs space-time coding on a plurality of transmission signals, which are then multiplexed, distributed to M antennas, and transmitted to a plurality of MIMO channels. The receiver performs space-time decoding on reception signals received by N antennas via the channels to obtain reception data. In this case, a channel model is composed of a radio wave environment around the transmitter (transfer function), a structure of channel space (transfer function), and a radio wave environment around the receiver (transfer function). In the case of multiplexing signals transmitted from each antenna, there occurs crosstalk. However, by signal processing at the receiver, each multiplexed signal can be extracted correctly without crosstalk.
As MIMO transmission schemes, a variety of schemes are proposed; however, how to exchange channel information between the transmitter and the receiver in accordance with an antenna configuration is a big issue for implementation.
In the case of exchanging the channel information, it is easy to perform a method of transmitting known information (preamble information) only from the transmitter to the receiver. In this case, the transmitter and the receiver are independent of each other to perform spatial multiplexing transmission, and this is called an open-loop type of MIMO transmission. As an extension of this method, there is a closed-loop type of MIMO transmission for producing an ideal spatial orthogonal channel between the transmitter and the receiver by feedback of preamble information also from the receiver to the transmitter.
The open-loop type of MIMO transmission can include V-BLAST (Vertical Bell Laboratories Layered Space Time) scheme for example (e.g., see patent document 1). The transmitter does not provide an antenna weighting factor matrix, and simply multiplexes and transmits signals for each antenna. In other words, a feedback procedure for obtaining the antenna weighting factor matrix is entirely omitted. The transmitter inserts training signals for performing channel estimation at the receiver, in a time-division manner, for example for each antenna, before transmitting multiplexed signals. On the other hand, the receiver performs the channel estimation using the training signals at a channel estimation unit and calculates a channel information matrix H corresponding to each antenna pair. By combing zero-forcing and canceling skillfully, a signal-to-noise ratio by utilizing a degree of freedom of each antenna that is caused by the canceling is improved and a degree of certainty of decoding is enhanced.
Further, as an ideal form for the closed-loop type of MIMO transmission, there is known a SVD-MIMO scheme utilizing singular value decomposition (SVD) of a propagation path function (e.g., see non-patent document 5).
In the SVD-MIMO transmission, UDVH is obtained by performing the singular value decomposition of a numerical matrix whose elements denote channel information corresponding to each antenna pair, namely the channel information matrix H, and the transmission antenna weighting factor matrix V and the reception antenna weighting factor matrix UH are obtained. Thus, each MIMO channel is expressed as the diagonal matrix D having the diagonal elements that are the square root of each eigenvalue λi, and signals can be multiplexed to be transmitted without any crosstalk. In this case, there can be realized a plurality of logically independent, space division (i.e., spatial orthogonal multiplexing) transmission channels.
It is generally assumed that, based on transmission channel information, the transmitter calculates an optimum antenna weighting factor and optimizes a coding rate and a modulation scheme that are applied to bit streams for each transmission antenna in the closed-loop type of MIMO scheme and thereby more appropriate information transmission can be realized. However, there is a problem that the closed-loop type of MIMO communication, in order to be introduced as a real system, needs higher frequency of feedback from the receiver to the transmitter in the case of a large channel fluctuation due to a move of the transmitter/receiver.
The open-loop type and the closed-loop type of MIMO communication schemes will be described below.
FIG. 7 schematically shows the configuration of a MIMO transmitter/receiver of the open-loop type. In this case, the transmitter transmits a data frame containing a preamble signal (TxPreamble) for estimating a propagation path. The receiver obtains a channel matrix H based on the received preamble signal and performs weighted reception of data division using a reception antenna weight calculated based on the channel matrix H. A feedback procedure is not performed from the receiver to the transmitter.
FIG. 8 schematically shows the configuration of a MIMO transmitter/receiver of the closed-loop type. In this case, the transmitter transmits a preamble signal (TxPreamble1) for estimating a propagation path. The receiver can obtain a channel matrix H based on the received preamble signal and sends feedback of a preamble signal (RxPreamble) for estimating the propagation path. The transmitter can obtain the channel matrix H based on the received preamble signal. Further, the transmitter obtains a transmission weight matrix based on the obtained channel matrix H and performs weighted transmission of a data frame to which a preamble signal (TxPreamble2) is attached. The receiver can obtain a new channel matrix based on the preamble signal (TxPreamble2) and performs weighted reception of a data frame using a reception weight matrix calculated from the channel matrix.
It is possible to exchange the preamble signals (TxPreamble and RxPreamble) between the transmitter and the receiver along with an RTS/CTS sequence for solving a hidden terminal. Further, the transmitter may transmit the preamble signal just one time. Furthermore, in the case where the reversibility of the propagation path is valid, the transmission may be made in order of RxPreamble to TxPreamble.
Thus, in both transmission schemes of the open-loop type and the closed-loop type, the receiver employs preamble information that estimates channel information of a certain time and demodulates the remaining data division.
In a communication system (such as the MIMO transmission scheme) for performing weighted transmission/reception based on the channel matrix H obtained from propagation path conditions, change over time by the channel matrix H becomes a problem. The channel matrix H changes every moment because of a change in, for example, room temperature or other atmospheres, a change in a reflected path due to a move of a person or a device, or the like. In particular, in the case of transmitting a large amount of data at a time, there occurs a channel fluctuation over time and the accuracy of the preamble information becomes deteriorated, so that the problem comes to the surface.
For example, in the processes of data transmission in the closed-loop type of MIMO communication system shown in FIG. 8, if transmission data is long, it is realistic that the transmitter inserts TxPreambles at fixed periods as necessary (see FIG. 9) and the receiver acquires channel matrices successively to establish communication. At this time, in the case of the closed-loop type, since the transmitter has established a transmission antenna weight, it is needless to say that the accuracy deterioration of the preamble information largely affects decoding characteristics.
[Patent document 1] Japanese Unexamined Patent Publication No. Hei 10-84324
[Non-patent document 1] International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part 1: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent document 2] ETSI Standard ETSI TS 101761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-patent document 3] ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer
[Non-patent document 4] Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band
[Non-patent document 5] http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf (as of Oct. 24, 2003)